by andrewa » Fri 25 Nov, 2016 7:44 pm
If your sleeping bag insulates you well, the outside of it will be at air temperature, which means that there's not enough temperature differential on the outside of the bag to "drive" any moisture from you through the bivy bag, so the outside of your your sleeping bag will get damp. This is why goretex sleeping bag outers don't work, and why the inside of goretex bivy bags get damp.
If the outside of your sleeping bag is warm ( meaning that your sleeping bag is not insulating you properly) then there might be enough temperature differential to push the vapor through the outer layer.
The balance point is where you are at perfect temperature, and don't produce any sweat/vapor, which means that there is no vapor to go through any outer bag , in which case the outer bag could be waterproof nylon, as could the inner lining - the latter being a vapour barrier.
I suppose the ideal bivy sack might just be a lightweight nylon with a decent DWR finish, which is really the ideal sleeping bag outer fabric.
A